Carburetor Calibration

It is true that out of the box Holley double pumper carburetors are calibrated differently than the vacuum secondary versions. The double pumpers do have a different metering block with a smaller power valve restriction and what this does is make you run a larger jet for the same full throttle power mixture which is the same with either carburetor. The vacuum secondary carburetor has larger power valve channel restrictions and this lets you use a smaller jet for the same full throttle power mixture but at part throttle the mixture is leaner than with the double pumper.

The reason for this is probably to make the double pumper carburetor have better drivability with an engine that has a performance camshaft. What I've found personally is that it tends to make the carburetor burn excessive fuel and run too rich at part throttle.

I think that the calibration of the idle circuit is close to identical on either carburetor.

In my case I was talking about some carburetors done by Bobby Oliver at Competition Carburetion. He does a really nice carburetor package in either a double pumper or a vacuum carburetor and he's taught me a few things about this stuff.

One trick on a Holley that I found works really well:

A customer had an 850 straight booster double pumper on a 454 Chevy in a jet boat. The engine makes 500 horsepower at 5000rpm and has a Comp 280 street roller cam(good combination). It was very rich in the transition circuit from off idle to nearly full throttle. Cutting down the idle feed restrictions in both blocks helped the fuel economy and overall drivability a TON! It was just way too rich in that area even with the wide open throttle mixture in good shape. Easy to fix and it saved a lot of fuel. I think he said that his cruise fuel consumption dropped by about 1/3! Not that jet boats are ever going to get great economy but why waste fuel?